From Libertarian Wiki

The porcupine is an animal that has been adopted by some libertarians as a symbol, most notably by the Free State Project. The porcupine is a cute and cuddly creature who is friendly but nonetheless armed and ready to defend himself if somebody initiates aggression against him. Since the adoption as a mascot by the Free State Project, other uses of the porcupine as a libertarian symbol have emerged.

The porcupine has seen scattered use before the Free State Project as a libertarian symbol too, such as by the Libertarian Party of Nevada.